1. Field of the Invention
The invention relates to dryer drum coaters and, more particularly, relates to a method and apparatus for recirculating blue smoke, formed upon the mixing of recycled asphalt products (RAP) with virgin aggregate, into the dryer drum burner flame for incineration.
2. Discussion of the Related Art
The mixing of recycled asphalt products (RAP) with virgin aggregate is now in wide use in the asphalt production industry. The virgin aggregate is typically first heated and dried in a rotary drum of a drum mixer or dryer drum coater and then mixed with RAP and liquid asphalt in a separate mixing chamber of the dryer drum coater to form a hot asphalt mix suitable for paving. One such dryer drum coater, manufactured by Astec Industries, Inc. and commonly known as a "double barrel dryer", is schematically illustrated in FIG. 1. The dryer drum coater 10 illustrated in FIG. 1 comprises an inner drum 12 and a fixed outer sleeve 14 mounted on a common frame 16 in an inclined manner. The inner drum 12 is rotatably mounted on the frame 16 by a plurality of bearings 18 and is driven to rotate by a suitable motor 20. A burner 22 directs a flame 24 generally axially into the interior of inner drum 12.
Inner drum 12 has at its first (upper) end 26 a virgin aggregate inlet 28 and a combustion products outlet 30, and has at its second (lower) end 32 a plurality of openings 34 forming heated and dried virgin aggregate outlets. Inner drum 12 also supports a plurality of paddles 36 extending into a mixing chamber 38 formed between the inner drum 12 and the outer sleeve 14. The interior of the inner drum 12 is functionally separated into a combustion zone located in the vicinity of the burner flame 24 and a drying zone located between the combustion zone and the first end 26 of the drum 12.
Outer sleeve 14 is separated from the inner drum 12 by a sufficient distance to form a mixing chamber 38 which is sufficiently wide to provide clearance for the paddles 36. Outer sleeve 14 has an upper RAP inlet 40, a virgin aggregate inlet 42 cooperating with the openings 34 of the inner drum 12, and an asphalt mix outlet 44. Outer sleeve 14 also receives suitable equipment (not shown) for injecting liquid asphalt into the mixing chamber 38.
In use, virgin aggregate is fed into the virgin aggregate inlet 28 of the inner drum 12 via a suitable conveyor 46 and is heated and dried as it travels downwardly through the inclined drum 12 counter to the direction of the flame 24 from the burner 22. Heated and dried aggregate in the second end 32 of the drum 12 falls through openings 34 in the drum 12, through the inlet 42 in the sleeve 14, and into the mixing chamber 38. RAP is simultaneously fed into mixing chamber 38 from the sleeve inlet 40 by a suitable conveyor 48 and is mixed by the paddles 36 with the heated and dried virgin aggregate. Liquid asphalt is also normally injected at this time, thereby forming an asphalt paving mix. In addition to mixing the virgin aggregate, RAP, and liquid asphalt, the paddles 36 also convey the resulting mix to the mixing chamber outlet 44, where the mix is discharged from the dryer drum coater 10. Combustion products formed during operation of the dryer drum coater 10 rise out of the inner drum 12 through outlet 30 and are conveyed to a downstream device such as a bag house.
Vapors laden with hydrocarbons and other contaminates are typically produced in the mixing chamber 38 upon the mixing of RAP and liquid asphalt with virgin aggregate. These vapors, commonly called and henceforth referred to as "blue smoke", are drawn through the openings 34 and into the interior of inner drum 12. Much of the blue smoke in the drum 12 flows along the shell of the inner drum 12 as represented by the arrows 52 and is discharged from the drum through outlet 30 without ever contacting the burner flame 24, thereby resulting in the emissions of relatively high quantities of undesired pollutants such as VOC's, NOx, etc.
During normal operation of the dryer drum coater 10, the momentum of the burner flame 24 results in a higher pressure region near the end of the flame 24 relative to a lower pressure region near the base of the flame. This pressure differential results in the formation of eddy currents 50 resulting in backward circulation of some of the combustion products. Eddy currents 50 may entrain a small portion of the blue smoke drawn out from the mixing chamber and brought into contact with the flame. However, these eddy currents are too small, too weak, and are located too near the end of the flame to result in significant blue smoke entrainment or significant reduction of blue smoke emissions.